Biological Nitrogen Fixation (BNF) may be the most important economic and environmental process affecting soybean production in the US. Yet, it is only partially characterized. We know surprisingly little about the amounts of N fixed by US soybeans.
The main question that researchers and producers are both asking is: Do high yielding soybeans need to be fertilized with N? Soybeans require a lot of N, 4 to 5 lbs. per bushel of grain yet BNF can only produce from 40 to 70% of what is required with the rest coming from the soil. As growers produce more soybeans in the 70 and 80 bushels range, or the environment becomes more stressful, both BNF and soil reserves will not be able to supply enough N and could become limiting, thus potential for supplemental N response will become more common.
Responses from applied N are inconsistent and unpredictable, and with not much information about the interaction with sulfur. In addition, a better understanding of the N balance for soybean is needed in order to sustainably increase production, maximize profits, increase the efficiency in the use of inputs and reduce the environmental footprint of current food production systems.
Lastly, studying the effects of this response on planting date and the end of the season, dry-down changes in the seed moisture, are relevant for farmers in this region..
For the entire project, data will be analyzed locally to determine response trends across broad scales. Results will be summarized and disseminated to growers via relevant extension outreach programs and peer-reviewed publications. Research results will be used to create extension fact sheets detailing Best Management Practices for soybean production.

This research aims to 1) quantify the effects of nutrition (considering N and S fertilization rates) under contrasting planting dates on yield and seed quality (protein, oil), 2) quantify effects of nutrition and planting date on overall nitrogen fixation process and budgets (sustainability), and 3) study differences in dry down until harvest time. Several soybean specialists and US states (+10 states) were involved during 2020 season, with our team from Kansas taking the lead on this initiative. A better understanding of quantities of N fixed and the soil, soybean management, and environmental factors affecting N fixation will contribute to the science leading to greater soybean yields, higher soybean quality, lower environmental impacts and sustainability, and greater returns to US farmers.

All project outcomes will be disseminated in diverse research and extension communication outlets to help Kansas soybean producers for maximizing financial returns and preserve the land and water resources under their control.

Update:
All trials were planted, data collected, and crop harvested during the 2022 growing season. Crop phenology data throughout the entire growing season and yield information for all nutrient treatments is presented in the attached report. Plant tissue data and N fixation data is currently analyzed and with the samples to be submitted to the further lab testing.

View uploaded report

Update:
All data from 2022 growing season has been processed and current 2023 studies already planted and under evaluation. Data on soil and weather, crop phenology, nutrient sampling, and nutrient concentrations, and soybean seed yields are presented on the attached final report from the 2022 growing season. For the explored locations, the yields ranged from 59 to 71 bushels/acre. Greater potential to sulfur fertilization response has been documented for all locations, but more clearly for Topeka. From the perspective of in-season crop nutrient characterization, it is clear that variations in potassium (K) and nitrogen (N) are clear within the canopy, while no clear changes were documented for phosphorous (P) and sulfur (S). These new results provide evidence for future changes in nutrient sampling to provide more precise diagnosis.

View uploaded report

The main outcomes from this study can serve to provide a new foundational knowledge to rethink in-season nutrient diagnosis and to focus on characterizing canopy changes in nutrient status, with a main focus on N and K changes during the growing season. From the yield perspective, yields ranged from 59 to 71 bushels/acre, with a greater potential to sulfur (S) fertilization response has been documented for all locations, but more clearly for Topeka site. An evidence that potential on yield response to this nutrient should be further explored on more sites, different soils and weather conditions across the region.

This project will provide useful information on the overall yield and seed quality impact of early- versus late- planting dates with the effect of nutrition. This project will provide new insights on the interactive effects of nutrition and environmental factors (e.g.,. planting dates) affecting yield and even more relevant for the future soybean quality.